1. Field of the Invention
The present invention relates generally to force sensors, and more specifically to an ultrasonic sensor for the measurement of normal and shear forces.
2. State of the Art
With rare exception, tactile or contact-type sensors in the art respond to normal forces only. From the measurement of normal force distribution, three (F.sub.z, M.sub.x, M.sub.y) of the six force-torque components (F.sub.x, F.sub.y, F.sub.z, M.sub.x, M.sub.y, M.sub.z) can be computed. These three components are the normal force and the two orthogonal torques in the plane of the sensor. Normal-force sensing is adequate for tasks involving object or feature identification, determining object location with respect to the sensor, and under some circumstances, estimating impending slip from the normal force and knowledge of the coefficient of friction between the object and the sensor surface.
However, for certain applications, such a limited sensing capability is inadequate. Examples of such applications include, without limitation, gasping and manipulation by a robot hand; measurement of forces generated by an object such as tire, shoe, boot or ski moving over the sensor; determination of pressure points, forces and movements of bodily extremities with respect to footwear such as athletic shoes, boots, and ski boots as well as sporting (golf clubs, tennis rackets, baseball bats) and industrial (hand tools, grips for electrically-powered tools) implements; determination of balance and gait analysis for athletic training and medical treatment and rehabilitation; use in a joystick, cursor control or other position-dependent control devices; and for accelerometers.
There have been several attempts to develop arrays of triaxial force sensors or full six-axis tactile sensors. For example, tactile array elements have been composed of magnetic dipoles embedded in an elastomer, the position and orientation of which were detected by magneto-resistive sensors. However, only one- or two-element sensors have been fabricated to prove feasibility of the concept. Another approach has employed sensors using emitters (charge or magnetic) embedded in a compliant layer. Emitter position is measured by an array of field-effect transistors or Hall-effect devices fabricated on a silicon substrate. Prototype sensors of this design were found to be highly sensitive to external fields.
A capacitance-based approach has also been attempted, but implemented only with respect to normal-force sensing. An existing, optically-based tactile sensor may have been modified to incorporate shear sensing capabilities. Presumably, the technique being investigated is the position monitoring of optical targets embedded in a substrate. However, such a design does not lend itself to incorporation into necessarily compact sensors as used in robot end-effectors, due, among other consideration, to the presence of a relatively large, stiff bundle of optical fibers exiting the sensor.
A miniature force-torque sensor has been developed by the assignee of the present invention. This sensor was intended for mounting on the gripping surfaces of robot end-effectors. The sensor consists of an elastomeric spring element joining two rigid parallel plates, one of which is mounted to the end-effector. Ultrasonic pulse-echo ranging through the elastomer is used to detect fine movements of one plate relative to the other. The sensor is compliant, the degree thereof as well as the sensitivity and load range of the sensor being alterable by changing the elastomer composition. The six force-torque components may be calculated from the transit times and specifically times-of-flight (TOF) of a plurality of differently-aimed pulse-echo signals as one plate is deflected with respect to the other under application of force. A further description of the aforementioned sensor appears in U.S. Pat. No. 4,704,909, assigned to the assignee of the present invention, and incorporated herein by reference.
Other force sensors developed by the assignee of the present invention, which sensors employ pulse-echo ranging, are U.S. Pat. Nos. 4,964,302 and 5,209,126, assigned to the assignee of the present invention and incorporated herein by reference. The sensors disclosed in these two patents do not, however, have triaxial force component determination capability.